In one embodiment, the present invention relates to optical transmission systems and more particularly to systems and methods for modulation.
In order to accommodate increasing demands from Internet traffic, optical communication links are evolving to carry higher and higher data rates over greater and greater distances. Wavelength division multiplexed (WDM) links are being developed to carry greater numbers of more densely spaced channels.
There are, however, obstacles to the further development of high capacity, long haul WDM transmission systems. Despite the use of advanced chromatic dispersion compensation techniques, residual uncompensated chromatic dispersion may still be present on certain WDM channels at the receiver, causing unwanted intersymbol interference. High data rate broadband modulation used over long distances is very susceptible to amplified stimulated emission (ASE) noise, which can cause receiver errors.
At the transmitter end, it becomes difficult to provide modulator and transmitter electronics with sufficient electrical bandwidth, peak to peak voltage swing, and slew rate to support increased data rates, e.g., greater than 10 Gbps. In order to overcome receiver susceptibility to noise, power levels may be increased. This exacerbates unwanted non-linear effects such as four-wave mixing, cross phase modulation and Raman crosstalk.
Yet another problem is posed by limited optical bandwidth. Optical bandwidth has been customarily viewed as being unlimited. Nonetheless, as the number of channels increases, the inter-channel spacing decreases and the bandwidths of the modulated signal increase as a result of increased data rates. The result is that co-channel interference becomes a concern.
The use of a LiNbO3-based device to provide phase modulation instead of the more widespread amplitude modulation has been proposed as a tool with which to address some of the above problems. This phase modulation scheme does not, however, substantially improve chromatic dispersion performance.
What are needed are systems and methods for practically implementing phase modulation in an optical digital transmission system to help address the above-identified obstacles to the further development of long-haul high data rate optical communication links, and in particular to address the problem of tolerance to chromatic dispersion.
By virtue of one embodiment of the present invention, a Mach-Zehnder interferometer is employed to implement phase modulation of an optical signal. The interferometer is biased at its maximum extinction point. The modulated optical signal may have a peak magnitude between approximately 30% and 80% of a maximum output power of the interferometer.
A first aspect of the present invention provides a method of phase modulating an optical signal. The method includes: driving a Mach-Zehnder interferometer with a data signal biased at a level approximately equal to a maximum extinction point of the Mach-Zehnder interferometer, and outputting the optical signal, phase modulated by the data signal, from the Mach-Zehnder interferometer.
A second aspect of the present invention provides apparatus for phase modulating an optical signal. The apparatus includes a Mach-Zehnder interferometer having a modulation drive input and a modulation driver that provides a data signal to the modulation drive input, the data signal being biased to a level approximately equal to a maximum extinction point of the Mach-Zehnder interferometer. The Mach-Zehnder interferometer outputs the optical signal as phase modulated by the data signal.
Further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.